1. Field of the Invention
The present invention relates to a display for displaying an image after converting an analog video signal to a digital signal.
2. Description of the Related Art
Apparatuses for displaying images, such as liquid crystal displays (LCDs) and LCD projectors, are connected to devices for generating image data, such as personal computers (PCs). Since signals output to the display apparatuses are generally analog red, green, and blue (RGB) signals, output levels of image data are different from one device to another. In other words, signal levels of image data output to the display apparatuses have variations. This makes it difficult to display a high quality image.
Japanese Patent Laid-Open No. 2003-131641 (in particular, paragraphs 54 to 66 and FIG. 1, corresponding to paragraphs 62 to 74 and FIG. 1 of U.S. application Publication No. 2003-0080985) discloses an image display system having automatic image adjustment capability. The system is described below with reference to FIG. 10.
FIG. 10 is a block diagram showing a method for adjusting an image in a conventional image display system, as mentioned above.
An image display device 101b includes a display unit 104, an analog-to-digital (A/D) converter 105, an automatic level adjuster 106b, and an adjusting-image data output instruction signal generator 109. An image data generation device 102b includes an image data sender 107b, a general image data generator 110, and an adjusting-image data generator 111.
A transmission unit 103b transmits an adjusting-image data output instruction signal and an adjusting-image data output completion signal for communication control and an analog image signal between the image display device 101b and the image data generation device 102b. 
In an automatic image adjustment process, an adjusting-image data output instruction signal is sent to the image data generation device 102b, and the image data generation device 102b sends adjusting image data together with an adjusting-image data output completion signal to the image display device 101b. Using the adjusting image data, the image display device 101b performs automatic level adjustment.
However, the image display system disclosed in Japanese Patent Laid-Open No. 2003-131641 has the following problems:    (1) In addition to an analog image data signal, an adjusting-image data output instruction signal and adjusting-image data output completion signal for communication control are required.    (2) In order to forcibly set an image suitable for automatic level adjustment, it is necessary to have communicating means for sending an adjusting-image data output instruction signal from the image display device 101b to the image data generation device 102b.     (3) In order to make sure that the image data generation device 102b outputs an adjusting image after receiving an adjusting-image output instruction signal, it is necessary that an adjusting-image output completion signal be sent to the image display device 101b.     (4) In addition to a signal line for analog RGB image signals, a signal line for communication control (e.g., a universal serial bus (USB) cable or Recommended Standard 232C (RS-232C) cable) is required. This is disadvantageous in terms of costs and cable connections. Also, it is necessary to create an image dedicated to image adjustment in the image data generation device 102b, and therefore, it is impossible to individually perform adjustment in different image data generation devices.